If you copy and paste from a web page - including a full table - into a GitHub issue comment GitHub converts it to the corresponding Markdown for you. Really quick way to construct Markdown tables.
https://domchristie.github.io/turndown/ is an open source JavaScript library by Dom Christie that can convert HTML strings into Markdown strings. Code: https://github.com/domchristie/turndown - it used to be called to-markdown.
https://euangoddard.github.io/clipboard2markdown/ is a tool which lets you paste in rich-text and uses turndown to convert it for you directly in your browser.
https://2md.ca/ is another tool that does this, written in TypeScript. It's accompanied by a detailed essay explaining exactly how it works.